Medical devices are used for myriad purposes on and throughout an animal's body. They can be simple ex vivo devices such as adhesive bandages, canes, walkers and contact lenses or complex implantable devices including pace makers, heart valves, vascular stents, catheters and vascular grafts. Implantable medical devices must be biocompatible to prevent inducing life threatening adverse physiological responses between the implant recipient and device.
Recently, highly biocompatible polymers have been formulated to provide implantable medical devices with coatings. These coatings not only increase an implant's tissue compatibility but can also function as bioactive agent reservoirs. However, designing polymer coatings for medical devices have proven problematic. All medical device coatings must be non-toxic, durable and adhere well to device surfaces. Additionally, when the medical device comes into intimate contact with unprotected tissues such as blood and internal organs it must also be biocompatible. Furthermore, if the medical device is designed to be pliable either in operation or deployment, the coating must resist cracking, fracture and delamination.
Moreover, medical devices intended to act as bioactive agent (drug) reservoirs must not only be biocompatible, structurally stable and resistant to delamination, but also chemically compatible with the drug to be deployed. Furthermore, if the reservoir is also intended to control the drug's release rate into adjacent tissue the polymer used must possess other highly specialized properties as well.
Presently, designing a biocompatible polymer coating having the desired physical and chemical properties has been largely a process of trial and error. Material scientists skilled in polymer chemistry make a preliminary polymer selection based largely on educated guesses. Next a series of experiments designed to establish the new coating composition's performance characteristics are performed and the results compared to an idealized model. However, very few potential polymer compositions will possess all of the desired properties required for a medical device controlled release coating. Consequently, present controlled release coating development processes are tedious, time consuming and seldom result in an optimized medical device coating having the combination of biocompatibility, ductility, surface adhesiveness and drug-polymer solubility.
Drug-polymer physical chemistry and the physical characteristics of the coating itself, such as coating thickness, are the two most important factors in determining a polymer matrix's drug elusion profile. Highly compatible drug-polymer combinations usually result in more even elution rates and are therefore preferable for most in vivo applications. Polymer-drug compatibility is a function of drug-polymer miscibility. The degree of miscibility, or compatibility, between a drug and a polymer carrier can be ascertained by comparing their relative solubility parameters. However, as will be more fully developed below, balancing drug elution rates with biocompatibility, ductility and adhesiveness requires more than merely matching a single polymer with a drug based on their total solubility parameters alone.
Therefore, it is an object of the present invention to provide medical device controlled release coatings made using a process that reduces trial and error and results in drug delivery systems having ideal physical and chemical properties.
Specifically, it is an object of the present invention to provide medical device coating systems that are flexible, do not delaminate from the device's surface, are highly biocompatible and provide for the controlled release of bioactive agents.